I am trying to re-style the expander of the TreeView. Is there a way to re-style the default expander (the triangle that turns orange on mouseOver) or do I have to completely re-vamp an entirely new template. It seems the only thing controlling this a ToggleButton. I'd like to keep the functionality that is present by default and it would help if I could just add some visual states on the "Expander". Let me know please, thanks.